Field of the Invention
The present invention relates to a capacitive micromachined ultrasonic transducer used as, for example, an acoustic wave conversion element and a test object information acquiring apparatus including the capacitive micromachined ultrasonic transducer.
Description of the Related Art
Micro mechanical members produced using micro machining technology can perform a micrometer-scale machining operation, and a variety of micro functional elements have been developed using such micro mechanical members.
Capacitive micromachined ultrasonic transducers (CMUTs) using such technology have been researched to replace piezoelectric elements.
Capacitive micromachined ultrasonic transducers have an actuator function for transmitting an acoustic wave by vibrating a membrane (a vibrating membrane) and a sensor function for receiving the acoustic wave reflected by a test object in the form of a displacement variation of the membrane.
By using vibration of the vibrating membrane of such a capacitive micromachined ultrasonic transducer, an acoustic wave can be transmitted and received. In particular, in liquid, excellent broadband characteristics can be easily obtained. As used herein, the term “acoustic wave” refers to a sound wave, an ultrasonic wave, or a photoacoustic wave.
An acoustic wave diagnosis apparatus transmits an acoustic wave from a capacitive micromachined ultrasonic transducer to a test object and receives a reflection signal from the test object using the capacitive micromachined ultrasonic transducer. Thereafter, the acoustic wave diagnosis apparatus obtains an acoustic wave image on the basis of the received signal.
Japanese Patent Laid-Open No. 2008-288813 (hereinafter referred to as a “Patent Literature 1”) describes improvement of the dielectric strength between two electrodes of a capacitive micromachined ultrasonic transducer applicable to acoustic wave diagnosis apparatuses.
The invention described in Patent Literature 1 is based on the finding that the dielectric constant of a silicon nitride film is higher than that of a silicon oxide film and the finding that a silicon nitride film easily accumulates electrical charge caused by a leakage current. More specifically, Patent Literature 1 describes a technology for increasing the dielectric strength voltage of an insulating film disposed between two electrodes that constitute a CMUT by disposing a portion of the insulating film formed from a silicon oxide film so that the portion is in contact with each of the two electrodes and disposing a portion formed from a silicon nitride film so that the portion is not in contact with the electrodes.